Single and double electron photodetachment from the oxygen anion at 41.7 nm

Single and double photodetachment of the oxygen anion O^- have been investigated at 41.7 nm (29.8 eV) in a crossed beams experiment using intense photon pulses from a free-electron laser. The ratio of single (O⁰+e^-) and double (O⁺+2e^-) detachment was determined to be σ_O⁰/σ_O⁺=4.12± 0.17 as identified directly from the yield of O⁰ and O⁺ fragments after irradiation. The absolute cross section for the dominating single detachment channel was measured to σ_O⁰=(2.1±0.6)×10^-19 cm². Analysis of photoelectrons detected in coincidence with neutral fragments (O⁰) suggests that single photodetachment primarily happens via the ground (³-0.16em0exP) or lowest excited (¹-0.16em0exD) state of oxygen. The results demonstrate the feasibility and advantage of crossed beams experiments for complete studies of photodetachment reactions.